Had a conversation with Siobhan. After reviewing the WordPress help tab content, it was clear that this TinyMCE help dialog content was duplicative and inferior. Everything here is already covered by the "Title and Post Editor" and "Inserting Media" tabs. So let's just axe the whole thing. The only thing that's useful here is the hotkeys, so let's rebrand the button as "Keyboard Shortcuts".

Ideally, this keyboard shortcuts dialog is a more complete, proper overlay like Gmail or Twitter (rather than be narrow and require scrolling). But this is fine for 3.9. We can give it a proper design in a future release and maybe even bring the same thing to the comments screen. Note for Gmail and Twitter you can use the shortcut ? (shift + /). We should absolutely adopt that for both screens too.